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Subject: Re: [PATCH 0/4] Reinstate and improve MIPS `do_div' implementation
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From:   "H. Nikolaus Schaller" <hns@goldelico.com>
In-Reply-To: <alpine.DEB.2.21.2104200044060.44318@angie.orcam.me.uk>
Date:   Wed, 21 Apr 2021 18:00:34 +0200
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Huacai Chen <chenhuacai@kernel.org>,
        Huacai Chen <chenhuacai@loongson.cn>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-arch@vger.kernel.org, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org
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References: <alpine.DEB.2.21.2104200044060.44318@angie.orcam.me.uk>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
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Hi,

> Am 20.04.2021 um 04:50 schrieb Maciej W. Rozycki <macro@orcam.me.uk>:
> 
> Hi,
> 
> As Huacai has recently discovered the MIPS backend for `do_div' has been 
> broken and inadvertently disabled with commit c21004cd5b4c ("MIPS: Rewrite 
> <asm/div64.h> to work with gcc 4.4.0.").  As it is code I have originally 
> written myself and Huacai had issues bringing it back to life leading to a 
> request to discard it even I have decided to step in.
> 
> In the end I have fixed the code and measured its performance to be ~100% 
> better on average than our generic code.

That would be good.

>  I have decided it would be worth 
> having the test module I have prepared for correctness evaluation as well 
> as benchmarking, so I have included it with the series, also so that I can 
> refer to the results easily.
> 
> In the end I have included four patches on this occasion: 1/4 is the test 
> module, 2/4 is an inline documentation fix/clarification for the `do_div' 
> wrapper, 3/4 enables the MIPS `__div64_32' backend and 4/4 adds a small 
> performance improvement to it.

How can I apply them to the kernel? There is something wrong which makes
git am fail.

> 
> I have investigated a fifth change as a potential improvement where I 
> replaced the call to `do_div64_32' with a DIVU instruction for cases where 
> the high part of the intermediate divident is zero, but it has turned out 
> to regress performance a little, so I have discarded it.
> 
> Also a follow-up change might be worth having to reduce the code size and 
> place `__div64_32' out of line for CC_OPTIMIZE_FOR_SIZE configurations, 
> but I have not fully prepared such a change at this time.  I did use the 
> WIP form I have for performance evaluation however; see the figures quoted 
> with 4/4.
> 
> These changes have been verified with a DECstation system with an R3400 
> MIPS I processor @40MHz and a MTI Malta system with a 5Kc MIPS64 processor 
> @160MHz.

I'd like to test on ~320 MHz JZ4730.

> 
> See individual change descriptions and any additional discussions for
> further details.
> 
> Questions, comments or concerns?  Otherwise please apply.
> 
>  Maciej

BR and thanks,
Nikolaus Schaller